The Yale graduate school regulations governing dissertations includes a paragraph that indicates that only two dissertation committee members need Yale appointments:

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“Registered doctoral candidates must have a principal adviser with an appointment on the Graduate School faculty. The Graduate School requires that each dissertation be read by at least three persons but not more than five, at least two of whom are ladder or ladder-track faculty members at Yale. All readers must hold the Ph.D. degree as well as a faculty position or be considered otherwise qualified to evaluate the dissertation…”
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